Currently, most television (TV) tuners employ narrow tracking filters near the their signal inputs which are used to diminish, or at least significantly limit, undesired portions of signals. Such filters generally are approximately 20 MHz bandpass filters and adjustable to pass a signal of interest. Placement of the filters near a signal input allows for a significant number of unwanted bands to be substantially removed before the signal reaches a demodulator as a narrow band signal. A narrow-band power measurement device at the baseband input to the demodulator is then used to provide Automatic Gain Control (AGC) by measuring the narrow band signal power and adjusting gain (usually at the signal input). The demodulator is usually located near the output of the signal path. While it might seem at first that detecting signal power near the output would allow signal power levels closer to the input to fluctuate without adequate control, the use of the narrow tracking filters can help to reduce power in a significant portion of the spectrum before the signal reaches amplifiers and other circuitry that are prone to distortion.
A disadvantage of narrow tracking filters is that they are not easily accommodated on semiconductor chips. In fact, with current technology, prior art systems have yet to implement narrow tracking filters on silicon because of the large inductors required in narrow tracking filter designs. Prior art systems that perform AGC by measuring narrow-band power near the output suffer from input power problems when narrow tracking filters are not present. For example, an RF signal input may include information on many channels, and the demodulator focuses on only one of those channels at a time. Thus, the narrow-band power measurement device adjusts the gain according to the measured power of whichever signal the demodulator is locked on to. However, the narrow-band power measurement device will not detect the power in the other channels. In other words, the AGC loop bases its determination on output power in the narrow band of interest but does not account for the full power spectrum of the input signal. As a result, high-power signals in those undesired channels are not controlled and can cause distortion in circuits that are close to the input, such as amplifiers.
One solution has been to implement narrow tracking filters separate from other parts of a tuner that are on a silicon chip. However, implementing large parts of a chip tuner off-chip can be expensive and defeats the purpose of integration. Further, implementing the narrow filters off-chip generally fails to provide an acceptable broadband input return loss, making such designs undesirable for cable applications. Currently, the prior art offers no substitute for narrow tracking filters. As a result, integration of tuner systems on chips has been impeded.
The AGC systems described above generally use analog attenuators to control the gain. Analog attenuators usually provide a continuous range of attenuation, and, therefore, a system can usually achieve a continuous range of signal power levels. A control system can then settle the signal power level at or very near a single reference power level. Engineers have been reluctant to try digital attenuators due to concern that discrete levels of attenuation offered by such attenuators would not allow a control system to settle, since there would be a small amount of error in a detected signal power level versus a reference signal power level. Accordingly, prior art AGC systems lack digitally-controlled attenuation.